Hi Jonathan,
I am a beer brewing rocketeer (just did a batch on tuesday) and can relate to your quest. I made a 1/2 barrel keg rocket wood stove conversion that works well as a hot liquor tank or brew kettle. But for real hot h20 making you need more surface area than just the bottom of a keg "barrel". So I built a tube shell heat exchanger but it is a bit of work,$, and more complicated. I added copper pipes in my mass bench so that I can extract hot h20 low tech for brewing or otherwise , or add hot h20 from my solar flat plate collectors to add heat to the mass. I think the pipes in a bench are a good low tech heat exchanger for lower temp heat (preheat) of brewing liquor. Rocket kettles are tall so must be built sturdy and stable even more so if you are going for a full three tier gravity system. Are you a welder? I think a bell with large enough copper pipes embedded in the top would be a sweet way to go for hot h20. Anyway, lots of ways to do it, just depends on your particular needs and $ and skills and time! My best advice, get rid of the plastic mash tun right away!
Canyon
